In shaping glass articles, molten glass gobs are fed to the individual shaping sections of the machines. The gobs are received in molds in which, either a parison is first formed by the press-and-blow or blow-and-blow process and then the final article is shaped by blowing, or else, the article is shaped directly in the molds.
The molds continuously receive gobs of molten glass and gradually increase in temperature. It is known that between the molds and the articles that are being shaped there is a heat compromise in that the molds must be kept at a temperature making it possible to maintain a heat balance between the glass and the mold. The molds must not be so hot that it additionally heats the glass to such a degree that the required stiffness for handle the articles cannot be achieved. On the other hand, the mold must not be so cold that it cools the glass and solidifies it more than necessary, impeding the suitable shaping of the article.
Heat control of the process of shaping glass and particularly glass bottles is very complex due to the number of variables including cycle time, mold temperature, weight of the articles and air temperature, which must be within certain limits to maintain good and uniform quality of the final product. Many decades of experience in the shaping process have established limits for each of the many variables. Each different article requires a different set of operating parameters which are based on prior experience in producing the articles and if an entirely new article is involved, operating parameters are established from prior experience with a similar article. This, together with high production demands, fluctuations in the cooling fluid temperature and the various designes of the shaping machines, cause the requirements for extraction of heat from the molds to vary widely. The molds are cooled, for the most part, by forced air which even though air has a low heat capacity and is not an ideal medium for heat transfer, it is however, more economical and easy to use in comparison with other cooling systems.
It is common practice in multisection machines to provide a single cooling air supply which is divided among the various sections of the machine and control of the total flow compensates for variations that occur in practice. It is important to compensate for these variations, since if they are not taken into consideration, the amount of cooling experienced by each particular mold changes, which leads to insufficient or excessive cooling and improperly formed glass articles that have to be thrown away. Two main sources of variation in the mold cooling system will be considered here.
A source over which it is not possible to exert effective control is the ambient air surrounding the machine because both the air temperature and its humidity change daily and seasonally. The other source comprises the condition of the cooling system such as the temperature and pressure of the cooling air supply, the properties of the cooling fluid such as humidity, viscosity, specific heat, conductivity, density, etc., and almost all the operating parameters of the machine, including type of glass handled, glass temperature, weight of glass gobs from which the articles are produced, temperature of the molds, operating time (cuts per minute), type of machine, etc.
Another important change in the cooling conditions under which the machine operates occurs from disturbances in the cooling air supply to the machine and/or when operating all or less than all the individual sections in a multisection machine.
Controlled cooling of molds in machines which produce glass articles, was originally done manually and was totally dependent on the judgment and experience of the operator. There was no analysis of causes and effects, and the results were far from optimal. Manual control resulted in a relatively high loss of articles and correspondingly a considerable reduction of production, because only when the operator realized that the articles did not have suitable characteristics, did he make adjustments which in his experience he considered suitable. There would be a preliminary loss of articles and after manual adjustments had been made and a period passed for the system to reach its heat balance, there would be a secondary loss of articles. Finally, when a more or less suitable heat balance was obtained, other disturbances occurred for different causes, such as variation of the furnace temperature that caused the glass to be too cool or too hot, a slight variation in the weight of the gob, and in general, internal and/or external heat changes (climatological changes) that required new adjustments. Thus, there was a third loss of articles until a state of equilibrium was attained. Continuous monitoring of the cooling by the operator with a considerable sacrifice in production were required which increased the manufacturing costs of the glass articles.
U.S. Pat. No. 3,860,407 of Fertik, issued on Jan. 14, 1975, describes a system for controlling the cooling of the molds in a glass forming machine which involves compensating the pressure set point for the control of the cooling air to correct for changes in the temperature of the cooling air and changes in the mass flow rate of the glass. In said system, the actual pressure and temperature of the cooling air are continuously compared with predetermined pressure and temperature set points for an established cooling process in order to compensate for variations from said predetermined set points.
The main problem with said system is that it is blind about the actual thermal exitation of the molds. In said system the pressure and temperature of the cooling air in the duct are continuously monitored but it does not take into account the actual physical conditions of the cooling air (depending of the physical conditions of the environment) which are of substantial influence in the thermal excitation of the molds. On the other hand, said system discloses that block 76 is a function of the speed of the machine and that the position of tab 86 a is proportional to weight of the glass gobs, but these data serve only to be compared with those of the predetermined set points in order to compensate for deviations from said set points. Concluding, the main problem of said system arises when the conditions of the cooling process could be at the desired set points, but the actual needs of said cooling process could be entirely different from said conditions established by the set points.
U.S. Pat. No. 3,953,188 of Fertik also, issued on Apr. 27, 1976, include some improvements over the above cited patent in that during conditions of maximum and minimum flow of the cooling air, the balance of heat transfer between the air and the parisons in modified by modifying the set points of the temperature controllers of the feeder or alternatively by changing the speed of the machine.
As in the previous mentioned system, in the above system the cooling process depends on the comparation of the actual conditions of pressure and temperature of the cooling air with those provided as set points, but including security means for saturation conditions.
U.S. Pat. No. 4,104,046 of McCreery, issued on Aug. 1, 1978, describes a system of automatic temperature control applied to a machine for continuous shaping of glass articles of the press-and-blow type. The conditions of heat transfer in the parison shaping units are automatically detected and the supply of cooling air, to these units is automatically controlled, to maintain uniform quality in the final shaped articles. In said system, the fluid passages are provided to make the cooling air circulate to the parison mold and the piston of each shaping unit and this air supply is controlled automatically in keeping with the heat transfer of these members. The control is automated by temperature detection instruments, such as infrared cameras placed in selected positions in the path of travel of the parison mold and piston of each of the shaping units of the machines. The cameras produce a signal from each member, which operates a control that, in turn, operates a pressure load regulator, such as an electropneumatic transducer in the cooling air supply line for that particular member. In this way, controlled temperature conditions are maintained to produce uniform articles, by avoiding excessive heating or cooling of the parison molds.
In said system the pressure of the cooling air is regulated depending on the temperature of the molds, but it does not take directly into account the actual exitation of the molds which depends also on the weight of the glass gobs and the velocity of the machine which determines the number of glass gobs processed by the molds. Furthermore it does not take directly into account the physical properties of the cooling air which are of substantial influence in the thermal excitation of the molds.
Further problems arise in said system because of the gases of oil and grease and the oxides appearing in the molds, all of which affect the measurements of the temperature of the mold cavities by the cameras.
U.K. Patent application GB No. 2011128 A, filed Dec. 1, 1978 and published July 4, 1979, describes a cooling control system to control the cooling of various parts of an independent section of a machine for shaping glass articles. The cooling air which goes through a cooling duct is controlled by measuring the cooling capacity of the air determined by placing a body whose material and surface characteristics approximate those of the molds in the air flow and measuring its surface temperature and simultaneously measuring the pressure of the cooling air supply. The flow of the air through the duct is increased or reduced to keep the pressure in the duct at a set point which depends on the temperature measured on the surface of the body located in the cooling air current.
That system also presents the same problem of the temperature set point established by the mold simmulator body.
As can be seen from the above disclosure, practically all of said systems are based on a constant thermal excitation applyed to certain fixed temperature conditions.
With this background, the present invention, trough an analysis of the requirements of the process for making bottles, defines the needs of:
1. Determining the actual thermal excitation of the molds in order to, through a predetermined model, if a variation on the thermal condition of entrance to the mold appear, the cooling system could compensate the cooling needs for said variations.
Applying the time series methodology on valuating the thermal excitation produced by the glass gobs to the molds, the time of response of the molds was determined. On this basis, if an indication of the velocity of the machine (through a gob cutting sensor) is obtained and if the thermal profile of the glass gobs (measured through a gob sensor placed at the output of the delivery equipment and at consecutive sampling) as well as the weight of the glass gobs (by weighing the finished articles or by some other signal available from the machine) are known, it is possible to accurate know the thermal excitation of the molds by adaptative control mathematic models.
2. Making the process of cooling the molds independent of the ambient variations by monitoring, at predetermined intervals of time, said ambient variations and conditions of the manufacturing process, for determining the physical properties of the cooling air and its cooling capacity for said manufacturing process conditions in order to compensate for said ambient variations.
To achieve the above it is first necessary to define a heat model in which the variation both of the ambient medium and the manufacturing process conditions would be related so that an automatic control could be performed that would have the enormous advantage of being sufficiently versatile, that the cooling process would be independent of the ambient conditions and manufacturing process.
As it is well known in tha art, theoretical research associated with the heat transfer mechanism by forced convection in cooling of molds in very complicated and so far it has not been possible to calculate a heat transfer coefficient by analytic methods.
However, the inventors here discovered that by applying the technique of dimensional analysis, the variables involved in the process of heat transfer by forced convection can be grounded in nondimensional terms to be able to find the heat transfer coefficient as a function of these terms, and by making a stable state energy balance, a heat model is obtained as a function of variables that are easy to measure and interpret.
In this way, with the method of dimensional analysis it was possible to establish an equation for the coefficient of heat transfer by forced convection, as a function of the variables involved in the process, and from the stable state energy balance, it was concluded that the molds and their parts acquire a temporary heat energy storage and that during the entire operation cycle they act solely as a transfer mechanism.
Thus, with te equation of the heat transfer coefficient and the energy balance equation, a heat model was reached whereby through a simple measuring system, operational parameters of the mold cooling system could be established.
The representative equation of the heat model discovered by the inventors of this invention is the following: EQU nlog P=(log .beta.*-jm)-(1+j.alpha.) log.(ts-Ta)
where:
.beta.*--is a factor that is a function of the conditions of operation and geometry of the machines (type of glass, amber, georgia green, etc., glass temperature, weight of article, type of mold, cycle time, geometry of nozzles, type of machine, blow pressure, no. of sections, lubricating, physical state of mold, etc.);
m--is a factor that is a function of the physical conditions of the cooling fluid (viscosity, density, temperature, humidity, etc.); PA1 n--is a factor that relates the air velocity with the static pressure (turbulence, nozzle losses, leaks, etc.); PA1 j--is a factor that is a function of the distribution of speed of the cooling fluid over the mold; PA1 p--is the static pressure of the cooling fluid; PA1 ts--is the temperature of the mold wall; PA1 Ta--is the temperature of the ambient air. PA1 T.sub.b.sbsb.t =is the deviation of T.sub.b.sbsb.t from its mean PA1 F.sub.2 (B) is a ratio of polynomials in the back order operator B and represents the transfer function due to P.sub.a.sbsb.t PA1 F.sub.1 (B) is the transfer fuction due to T.sub.a.sbsb.t and PA1 N.sub.t is the noise given by an ARMA (p, q) model.
This equation describes the process of cooling of molds by forced convection and expresses what the best way will be to achieve an efficient cooling of the molds since it involves in the first place the properties of air as the cooling fluid in its coefficients m and .alpha..
To obtain coefficients .alpha. and m, it is known that the cooling capacity of the cooling fluid (air) depends on its density .rho. and its viscosity .mu., but, these are a function of the temperature and relative humidity of the air, at a particular atmospheric pressure. We can therefore, graph on a logarithmic scale .rho./.mu. against a temperature representative of the mold surface (ts-Ta), which provides us a series of points that fit a straight line whose slope represents value .alpha. and the intersection of the line with the axis of the ordinates (axis .rho./.mu.) represents value m.
From a dimensional analysis, it is known that: EQU Nu=(A(Cpu).sup.f /K) (.rho.DV)j/.mu.)=APr.sup.f Re.sup.j =C.sub.1 Re.sup.j
Where Nu is the Nusselt number, Pr is the Prandtl number and Re the Reynolds number. If constant A and the Prandt approximate a constant C.sub.1 due to normal atmospheric conditions, the range of variations is negligible.
Therefore, in this equation, factor j depends on the geometry of the mold, nozzles, distance between them, etc.
To quantify j we measure the air flow and its static pressure and find the value of n by an equation: EQU Q=C.times.p.sup.n
Where Q is air flow, C is a constant that depends on the geometry of the ducts, pressure drops, etc., P is the static air pressure in the duct and n is a factor that relates the air velocity to its static pressure. Therefore, if we graph static pressure P against Ta-(ts-Ta), a straight line is obtained whose slope defines the value of (1+J)/n and hence J is quantified and the intersection of said line with axis ts-Ta, gives a value that corresponds to (log-.beta.*-Jm)/(1+.alpha.j) and, since .alpha., j and m are known, it is possible to know .beta.* which is the factor that is a function of the operating conditions and geometry of the machine.
It is further concluded that high value of .beta.* imply mainly high production rates, heavy articles or high glass temperature or a combination of them; these high values of .beta.* require a better design of the nozzles (parameter j), less air leaks or losses by friction (parameter n), etc.
Then parameter .beta.* will be a function of the manufacturing "history" mainly of the article and parameter j/n will refer to the design of the machine from the cooling viewpoint.
Tests run at the K-2 plant in Monterrey, N.L., MEXICO (I-S machines, 2 sections, acticles 50 grams, 23 cuts per minute) proved the viability of this heat model.
Thus, this invention teaches how to achieve a heat model which relates the conditions of the cooling fluid with the data of operation, modes of operation, production and type of shaping machine. Such a heat model makes it possible to provide a cooling system which is absolutely flexible and automatic so that cooling of the actual mold is made independent of variations of the ambient medium. On the basis of the heat model and keeping in mind the need of an automatic control, it is possible to provide an electronic system for controlling cooling of molds. Accordingly by detecting the physical conditions of the cooling fluid such as temperature and humidity and its static pressure, at predetermined intervals of time, it becomes possible--by a program that contains an equation as discussed above contained in the computer memory and through other suitable means--to achieve an effective control necessary in cooling of the articles and/or molds, whereby the quality of the articles produced and/or production of the shaping machines are improved.
Although the system of this invention for cooling molds includes the heat model described above, it is possible to include other types of heat models.
3. Having a forecasting model for predicting the changes in the physical conditions of the ambient air and the pressure of the cooling air in the duct needed for said predicted conditions, by measuring said actual temperature, humidity and pressure at predetermined intervals of time, for example each 2 hr. (at difference with Fertik's system which needs the continuous monitoring thereof), for establishing the ambient behavior history on which said forecasting is based and for correcting, if necessary, said parameters of the forecasting model which will be up-dated self-correcting the initial values gave to said model.
With this characteristic, the cooling system is independent of the temperature, humidity and pressure detections because if one or more of the sensors fail, the system will be working permitting the personal to change the defective sensors.
For said forecasting model, data measurements obtained in-line on a glass forming machine under a plant environment and consisting of cooling air pressure Pat, ambient air temperature Tat, and the external mold surface temperature T.sub.bt, were analyzed using discrete time series based on the Box Jenkins method. From the analysis, a dynamic model is given for predicting the values of the mold surface temperature T.sub.bt. The accuracy of the forecast model is corraborated with actual data. A control equation is derived for making the necessary adjustments of P.sub.at, and compensations for variations in T.sub.at, to ensure the regulation or minimum deviations, of the output T.sub.bt, from the target set point.
The analyzed data were taken at discrete time intervals of 2 minutes under normal operation conditions. The total sampling time being 9 hours and the production container 12 OZ. at an I.S. double cavity machine (57 bpm) located at our plant, VIQUESA, in Queretaro, Mexico.
Since T.sub.at is an observable variable and not being able to be manipulated, perturbations were induced in P.sub.at in order to know the dynamic response of the system (T.sub.bt).
A preliminary analysis of the raw data showed that T.sub.b.sbsb.t could be well represented by an ARMA (p, q) model whose spectral density function had two well defined spikes at values of about 6 minutes and 2 hours. The first one being due to the proper opening-closing function and the second, not clearly identified, is believed to come probably from batch charging operation, inherent viscosity variations, gob weight variations or some action taken in the heating/cooling system during the stages of melting, refining or conditioning.
In order to get rid of the complex arma structure, the data was smoothed by taking averages (each 3 observations) and the generated new data was used to build the dynamic model.
Using the smoothed data the identification and estimation stages were made and conducted to several possible models. Among them three were selected for further study since their residual sum of squares, noise autocorrelation and transfer function weights showed that the data were well represented by them.
Two of the models have the general form: EQU T.sub.b.sbsb.t =F.sub.2 (B)P.sub.a.sbsb.t +F.sub.1 (B)T.sub.a.sbsb.t +N.sub.t ( 1)
Where:
The third model have the same basic structure as (1) but the variables are expressed as deviations of their values at time t from those at time t-1, and the noise model having a MA (q) structure, however in factorizing this noise model in order to have deviations in N.sub.t, the deviations will be cancelled and arrive to equation (1). The basic difference among them is in the values of the transfer fuction and noise model parameters, which were estimated at the 95% confidence limits. The control equation has already been implemented in a microprocessor and its adequancy is being checked in-line, and is relatively easy to implement in a microprocessor due to the linear structure of the model.
4. And the need to achieve a control capable of totally automatic but with a manual operation capability, higly flexible in its operation and with a high reliability index. It is easy to operate, with low maintenance, with monitoring of the process information, economy in the process and the other parameters inherent in these requirements.